1. Field of the Invention
The present invention relates generally to an improved temperature difference measurement device and method and more specifically to a temperature difference measurement device and method using a temperature sensitive element which eliminates the adverse effects of offset and inherent characteristic unevenness and the variation in the temperature sensitive element and measurement device itself caused by aging which are detrimental to the accuracy of the device's measurements.
2. Description of the Prior Art
A conventional temperature difference measurement device, a part of which is shown in FIG. 1, comprises: (a) a temperature difference sensor, such as a thermocouple; (b) a processing circuit 1 having a plurality of operational amplifiers OP.sub.1, OP.sub.2 and OP.sub.3 and resistors R.sub.0 through R.sub.6 which receives and amplifies the signal at terminals V.sup.+ and V.sup.- that was generated by the temperature difference sensor, e.g., a thermoelectromotive force in response to a temperature difference, and outputs a voltage E corresponding to the measured difference in temperature; and (c) a measuring instrument (not shown in FIG. 1) which displays the measured difference in temperature, e.g., by means of a pointer, in response to the output voltage value E of the processing circuit 1.
The processing circuit 1 described above comprises: (a) a first operational amplifier OP.sub.1 having a noninverting input terminal (+) connected to one terminal V.sup.- of the thermocouple to receive the thermoelectromotive force generated by the thermocouple and connected to a constant voltage supply V.sub.0 via a first resistor R.sub.0, an inverting input terminal (-) connected to the constant voltage supply V.sub.0 via a second resistor R.sub.1, and an output terminal connected to the inverting input terminal via a third resistor R.sub.2 : (b) a second operational amplifier OP.sub.2 having a noninverting input terminal (+) connected to the other terminal V.sup.+ of the thermocouple to receive the thermo electromotive force generated by the thermocouple, an inverting input terminal (-) connected to the output terminal of the first operational amplifier OP.sub.1 via a fourth resistor R.sub.3, and an output terminal connected to the inverting input terminal thereof (-) via a fifth resistor R.sub.4 ; and (c) a third operational amplifier OP.sub.3 having a noninverting input terminal (+) connected to the output terminal of the second operational amplifier OP.sub.2, an inverting input terminal (-) connected to ground via a sixth resistor R.sub.5, and an output terminal connected to the inverting input terminal (-) via a seventh resistor R.sub.6.
In the processing circuit 1 described above, if a ratio of each resistor is expressed as .alpha.=R1/R2=R4/R3, .beta.=R6/R5, and if w.sub.1, w.sub.2, and w.sub.3 are respective offset voltages of the first, second, and third operational amplifiers OP.sub.1, OP.sub.2, OP.sub.3, the output voltage .mu..sub.1 of the first operational amplifier OP.sub.1 and the output voltage .mu..sub.2 of the second operational amplifier OP.sub.2 are expressed, respectively, in the equations: EQU .mu..sub.1 =(1/.alpha.)(V.sup.- -V.sub.0 +w.sub.1)+V.sup.- +w.sub.1 ( 1) EQU .mu..sub.2 =.alpha.(V.sup.+ -.mu..sub.1 +w.sub.2)+V.sup.+ +w.sub.2 ( 2)
In addition, the output voltage E of the processing circuit 1 is expressed in the equation: EQU E=(1+.beta.) (.mu..sub.1 +w.sub.3) (3)
From these three equations (1), (2), and (3), the output voltage E can be rearranged so that: EQU E=(1+.alpha.)(1+.beta.)(V.sup.+ -V.sup.-)+(1+.beta.){(1+.alpha.)(w.sub.2 -w.sub.1)+w.sub.3 +V.sub.0 } (4)
wherein (1+.alpha.) and (1+.beta.) are amplification factors of the associated operational amplifiers.
In the equation (4) expressed above, the first right-hand item indicates a theoretical amplified voltage value corresponding to the thermoelectromotive force produced by the difference in temperature, and the second right-hand item indicates an error voltage including each offset error voltage of the operational amplifiers. It should be noted that because of unevenness in thermoelectromotive force characteristic of each thermocouple, the first right-hand item inherently includes yet another offset error voltage factor such that the difference between the positive and negative sides of the thermocouple (V.sup.+ -V.sup.-) is not zero even when the temperature of both junctions of the thermocouple is 0.degree. C.
Nonetheless, conventional temperature difference measurement devices simply amplify the absolute value of thermoelectromotive force produced by the thermocouple, .vertline.V.sup.+ -V.sup.- .vertline.. This ignores the fact that the operating characteristic of the processing circuit 1 changes depending on the value of the circuit parameters .alpha., .beta., and V.sub.0 described above and the respective offset voltages w.sub.1, w.sub.2, and w.sub.3 of the first, second, and third operational amplifiers OP.sub.1, OP.sub.2, and OP.sub.3 which are determined by the operating characteristics of each circuit component used in the processing circuit 1.
In summary, the following three factors cause the change in the measured result of a difference in temperature: (x) an unevenness and drift in an output characteristic of the thermocouple used for the temperature sensitive element caused by aging; (y) the unevenness and dispersion in set circuit parameters built in during mass production of the devices; and (z) an unevenness and variation in the output characteristic of the circuit components caused by aging. In particular, the factor described at (y), which includes the effect of the offset voltages described hereinabove readily produces large errors in the output voltage E of the processing circuit 1. Consequently, measurement of a difference in temperature by such devices is likely to be inaccurate.